Polyhydroxyl compounds have attained great technical importance in numerous fields. For example, they are used on a large technical scale for the production of non-ionic surface active compounds, as anti-freezes, moisture-retainers and anti-hardeners and as starting components for the production of synthetic resins, such as polyester and polyether resins.
Polyhydric alcohols are at present obtained from natural products, such as sugar or cellulose materials, or synthesized by oxidation of petroleum derivatives.
In view of the world food situation, it appears unsuitable to use natural substances as raw materials for industrial products if they may be used as sources of dietary carbohydrates. On the other hand, due to the shortage of sources of petroleum, there has been a constant increase in the price of products which are dependent upon petroleum. Moreover, there are many uncertainties regarding the long term supply of petroleum products. It would therefore be desirable to find processes for the manufacture of polyhydroxyl compounds which do not depend for their supply of raw material on naturally occurring substances and petroleum.
Since the work by Butlerow and Loew (Ann. 120, 295 (1861) and J. pr. Chem. 33, 321 (1886)) in the previous century, it is known that condensation of formaldehyde hydrate (the term "condensation of formaldehyde" used in this text should be understood to mean in all cases "condensation of formaldehyde hydrate with itself") under the influence of basic compounds, such as calcium hydroxide or lead hydroxide, leads to the formation of hydroxyaldehydes and hydroxyketones. Since formaldehyde may be obtained from coal or natural gas by way of methanol, this would, in principle, be one way of obtaining hydroxyl compounds which would not depend on the supply of petroleum. Polyhydric alcohols could be synthesized by electrolytic reduction or catalytic or chemical hydrogenation.
In spite of numerous proposals for the synthesis of polyhydroxyl compounds by condensation of formaldehyde, however, no technically viable process has yet been developed for this synthesis because no one has yet succeeded in synthesizing mixtures of polyhydroxyl compounds in which the hydroxyl functionality is precisely reproducible. Moreover, the known processes result in the formation of hydroxyaldehyde and hydroxyketone mixtures which may only be hydrogenated with difficulty and only with the use of very large quantities of catalysts. This high catalyst consumption has hitherto made the synthesis of polyhydroxyl compounds by condensation of formaldehyde hydrate appear uneconomical. This has prevented condensation of formaldehyde hydrate from being used as the basis of a technical process for the synthesis of polyhydric alcohols.
Due to the disproportionation of formaldehyde into methanol and formic acid which takes place at the same time, only moderate yields have hitherto been obtained by the known processes so that working-up of the aqueous or aqueous alcoholic solutions obtained was very expensive.
It is well known that disproportionation of formaldehyde into methanol and formic acid is powerfully catalyzed by basic compounds. As was found by Pfeil, Chemische Berichte 84, 229 (1951), the reaction velocity of this so-called "Cannizzaro" reaction depends on the square of the formaldehyde concentration, while the reaction velocity of formaldehyde polyaddition (C--C linkage) depends on the formaldehyde concentration in a linear relationship (Pfeil and Schroth, Chemische Berichte 85, 303 (1952)). As the aldehyde concentration increases, therefore, the proportion of the desired polyhydroxyl compounds to methanol and formic acid obtained is shifted in favor of the unwanted compounds. It is for this reason that most of the known art processes propose to carry out the condensation of formaldehyde to hydroxyaldehydes and hydroxyketones in solutions which have a low formaldehyde concentration with a view to keeping the quantity of by-products as low as possible. However, the water used as solvent must then be removed by distillation in order to obtain the hydroxyaldehydes and hydroxyketones formed. This entails considerable energy costs due to the high heat of evaporation of water. Processes for the condensation of formaldehyde from dilute aqueous solutions are therefore uneconomical. Moreover, if distillation is prolonged, decomposition and discoloration reactions of the hydroxyaldehydes and hydroxyketones formed take place to a considerable extent.
It would therefore be desirable to carry out the condensation of formaldehyde from commercially obtainable concentrated formalin solutions without unwanted side reactions. In German Pat. No. 822,385 a process for the preparation of aliphatic hydroxy aldehydes is described in which a 40% formalin solution is reacted with thallium or thallium hydroxide. This process, however, is undesirable because of the toxicity of thallium and the fact that thallium hydroxide is difficult to obtain. Furthermore, the yields of this process are relatively low, being only from 70 to 80%.
Still with a view to preventing the Cannizzaro reaction, it has also been proposed to react formaldehyde solutions with calcium hydroxide or lead hydroxide in the presence of methanol, ethanol or other polar organic solvents (German Pat. No. 830,951 and Gorr and Wagner, Biochemische Zeitschrift, 262, 361 (1933)).
However, the addition of organic solvents again reduces the formaldehyde content of the solution. This process therefore also seems uneconomical on account of the additional energy costs required for evaporating off the added solvent to work-up the hydroxyaldehydes and ketones formed. Moreover, formaldehyde and lower alcohols give rise to unstable semiacetals which decompose under the conditions of the condensation reaction, with spontaneous liberation of the alcohols. Vigorous boiling phenomena therefore occur in the course of condensation reactions which are carried out at reaction temperatures above the boiling point of the given alcohol, particularly if large quantities of reaction mixture are used. These condensation processes cannot be carried out on a production-level without danger under such conditions.
A process for the preparation of oxy-oxo compounds has been described in German Pat. No. 884,794, in which aqueous formaldehyde solutions at concentrations of up to 30% are reacted with lead oxide or lead acetate and inorganic bases to form sugar-like compounds which reduce Fehling's solution in the cold. In this process, however, the formaldehyde solution must be heated for from 7 to 8 hours. The volume/time yield obtained therefore is not at all satisfactory. The relatively low yields (about 80%, based on the quantity of formaldehyde put into the process) are also by no means satisfactory.
A process for the preparation of hydroxyaldehydes and hydroxyketones has been disclosed in U.S. Pat. No. 2,224,910, in which exothermic condensation of formaldehyde is regulated by controlled addition of inorganic or organic bases to a formaldehyde solution containing compounds of lead, tin, calcium, barium, magnesium, cerium or thorium and a compound which is capable of enediol formation, such as glucose, ascorbic acid, fructose, benzoin, glycol aldehyde, erythrose, reductone, invert sugar or condensation products of formaldehyde. Although a mixture of hydroxyaldehydes and hydroxyketones is obtained from more highly concentrated formaldehyde solutions by this process without the addition of organic solvents, this is achieved only at the expense of various advantages. Thus, if the reaction is carried out at low pH values, the products obtained are mainly hydroxyaldehyde and hydroxyketone mixtures having a low hydroxyl functionality. Moreover, only moderate reaction velocities are achieved at low pH values so that the volume/time yields of this embodiment of the process are not satisfactory. To overcome these disadvantages, it is recommended in the above reference to start formaldehyde condensation at low pH values and then complete it at higher pH values. However, at pH values .gtoreq.7, lead-catalyzed formaldehyde condensation is so rapid, spontaneous and uncontrolled that mixtures of hydroxyaldehydes and hydroxyketones cannot be obtained with reproducible distribution of the components because the reaction times and conditions may no longer be accurately controlled. Furthermore, it is known that hydroxyaldehydes, hydroxyketones and monosaccharides decompose into dark colored, partially carboxyl-containing compounds in an alkaline medium at elevated temperatures.
These decomposition reactions are most pronounced in the preferred embodiments of the process according to U.S. Pat. No. 2,224,910, particularly when the major proportion of formaldehyde has already undergone reaction. Hydroxyaldehyde and hydroxyketone mixtures of the type prepared by the process according to U.S. Pat. No. 2,224,910 therefore contain decomposition products which have acid groups and the mixtures are brown in color and cannot be obtained reproducibly. Moreover, hydrogenation of these mixtures succeeds only with uneconomically large quantities of Raney nickel caatalyst. Hydrogenation of a mixture of hydroxyaldehydes and hydroxyketones equivalent to 100 g of formaldehyde requires 30 g of Raney nickel.
The product mixtures obtained by the last-described method must, in all cases, by worked-up by distillation for purification and for recovery of hydroxyl compounds of low molecular weight. It would, however, be desirable to dispense with the distillation of the mixture, which requires additional costs in energy and apparatus, and to obtain the product mixtures in such a way that they are ready for use as soon as the water of solution has been removed, without being first distilled. Such colorless reaction mixtures, substantially free from by-products are not obtainable by processes known in the art.
It is therefore an object of the present invention to provide a process for the synthesis of mixtures of polyhydroxyl compounds which are as far as possible free from decomposition products and which may easily be hydrogenated to polyhydric alcohols using small quantities of hydrogenation catalysts. The mixtures of polyhydroxyl compounds obtained should be colorless and require no further purification.
It is a further object of the present invention to control the condensation of formaldehyde so that the distribution of products in the resulting mixtures of low molecular weight polyhydroxyl compounds could be reproducibly varied as required.
German Pat. No. 881,504 generally discloses the use of formose (a mixture of hydroxyaldehydes and hydroxyketones obtainable by the condensation of formaldehyde) as a co-catalyst for the condensation of formaldehyde under slightly acidic reaction conditions. The patent discloses the use of a number of metal catalysts including lead oxide and lead hydroxide, however, it does not teach or suggest the combination of features which is critical for the process of the present invention.